“A couple of years ago, an enterprising New York chef made headlines by serving cheese made from his wife’s breast milk. His stunt provoked a swift reaction from the New York Health Department, which didn’t seem to find this novelty menu item amusing. Despite his venture’s unsuccessful end, a London ice cream parlor tried to imitate him last year by selling a breast-milk-based product.
Do you find the idea of human cheese and ice cream bizarre? Disturbing? If so, it’s not very difficult to see why. What’s more difficult to explain is how we came to drink milk from other mammals in the first place.
This might seem like an odd point to make, because for most Americans dairy is standard fare. But this is just one more thing that’s strange about us, because the majority of adult humans are actually lactose-intolerant to a greater or lesser extent. Taking the world population as a whole, lactose intolerance in adults is the norm. Lactose tolerance, by contrast, is only common in regions that historically practiced dairy farming — among people of European descent, for example. And this is no mere coincidence; for the rise of lactose tolerance is a striking illustration of evolution in action.
Lactose is a sugar just like sucrose, and in fact it has the same chemical formula: C12H22O11. But the structure is different. Put them side by side and it’s obvious:
Like sucrose, lactose is a marriage of two simple sugars. When you lactate, cells in your mammary glands synthesize lactose with the aid of an enzyme called lactose synthase; and the same is true for the mammary glands of a cow. As the primary sugar in milk, lactose is an important source of energy for both babies and calves alike. But to digest it they need to break it up into glucose and galactose, and since lactose and sucrose have very different structures, the sucrase enzyme that breaks up sucrose can’t do the trick. That’s why babies (and calves) make another enzyme called lactase instead.
Lactase breaks the lactose down in the small intestine so the baby can take up the glucose and galactose and use them for energy. Since lactose is found only in milk, however, there’s no point in continuing to make lots of the lactase enzyme into adulthood. It would be a waste of energy.
So as the baby grows older and moves on from milk to “real” food, in most cases the gene that codes for the lactase enzyme gets switched off and is no longer expressed at high levels. If you try to drink milk and your body is no longer producing the lactase enzyme, lactose passes through your small intestine undigested. When it gets to your large intestine, however, bacteria like E. coli are more than happy to chow down on these energy-rich leftovers, producing copious amounts of gas in the process. Variations in the composition of your gut flora — i.e., in the different proportions of various bacteria you have in your gut — may help determine the severity of your symptoms.
So most mammals don’t keep on drinking milk after they’re weaned. But some people, primarily those of Northern European descent like myself or descendants of certain African tribes, have inherited a mutant version of the lactase gene that remains active into adulthood. These folks continue to produce the lactase enzyme and can thus keep on digesting lactose with no problem….”